This invention relates to a method of and means for filtering environmental noise from speech, and more particularly noise that is near-stationary and of relatively long duration.
Environmental noise is often tolerated by persons with unimpaired hearing with no more discomfort than annoyance at the existence of such noise and the loss of ability to understand speech in the presence of such noise. For persons with impaired hearing fitted with a hearing aid having a fixed frequency spectrum, environmental noise is often disturbing, often interferes with their ability to understand speech, and is sometimes physically painful.
Environmental noise can be classified as follows:
(1) RELATIVELY SHORT DURATION NOISE SUCH AS CLICKING OF SHOES DURING WALKING, OR DISHES DURING STACKING;
(2) RELATIVELY LONG DURATION NOISES HAVING NEAR-STATIONARY SPECTRAL CHARACTERISTICS SUCH AS THE NOISE ASSOCIATED WITH PASSING CARS, TRAINS AND AIRPLANES, OR RUNNING FANS OR MACHINERY; AND
(3) RELATIVELY LONG DURATION NOISES THAT LACK STATIONARY SPECTRAL CHARACTERISTICS SUCH AS A BACKGROUND CONVERSATION, ETC.
The last mentioned class of noise may mask speech preventing its being understood by a hearing aid user and is disturbing for this reason. However, this type of noise does not assault the user's ear as do clicking noises and near-stationary noises of relatively long duration.
A relatively satisfactory solution to the problem of clicking noises is obtained by incorporating automatic gain control (AGC) into the circuitry of the hearing aid. Such circuitry responds to a sudden, high volume click, by automatically reducing the volume for the duration of the click thus suppressing the input to the user's ear. This eliminates not only the sound of the click, but any intelligence occuring simultaneously with the click. No loss of intelligibility of speech occurs, however, because of the short duration of the gain reduction and the ability of the ear to fill in a relatively short information gap.
So far as is known, however, no practical solution exists to the problem encountered with hearing aids due to near-stationary noise of relatively long duration. For the purpose of this description, the term "near-stationary, relatively long duration noise", hereinafter referred to as noise of the type described, refers to noise having particular time and spectral characteristics, namely noise of a duration exceeding about three seconds and whose frequency spectrum does not vary with time or varies only within a narrow range. As indicated above, vehicular and machinery noises are examples of noise of the type described. The minimum duration of noise of the type described is thus considerably longer than intervals of normal speech occuring between speech pauses.
It is therefore an object of the present invention to provide a novel method of, and means for, filtering noise of the type described from speech, whereby hearing aids or other devices can be adapted to operate properly under changing environmental noise situations.